1. Field of the Invention
The invention relates in general to the field of sound reproduction and, in particular, to damping systems for tone arms and cartridges used in the reproduction of sound recorded on phonograph records.
2. Prior Art
It is essential in the faithful reproduction of sound recorded on a phonograph record that the stylus be kept in proper contact with the grooves of the record so that it can follow the signal modulations in such grooves. Some of the audible indications that the tone arm is deficient in permitting the stylus to follow such modulations are: breakup and/or sibilance during massed string, brass or choral passages; lack of definition during loud or impulsive passages; a wandering or unstable stereo image; improved sound quality when a cartridge is tracked at or above the high end of its manufacturer-recommended tracking force range; and distortion (as a consequence of extreme woofer movement, intermodulation, or transient amplifier overload) resulting from large subaudible signals being fed into the input of the amplification/reproduction chain.
These audible deficiencies usually result from an uncontrolled resonance in the tone arm-cartridge-record system. The resonance for typical tone arm-cartridge combinations occurs in the range from 5-14 Hz. Unfortunately, virtually all commercial records are somewhat warped, having the highest relative warp occurrence, significant warp velocity and the large warp amplitude in the 5-14 Hz region. Since tone arm cartridge resonance occurs in the same frequency region as do most record warps, these warps thus excite this resonance with the result that the motion of the tone arm/cartridge combination can be many times larger than the motion of the stylus and the hence electrical output from the cartridge will be many times larger than that representative of the motion of the stylus in response to intentional signal modulations. Furthermore, since the tracking force (as measured between the tip of the stylus and the surface of a record) is the only force available to (a) accelerate the mass of the stylus tip relative to the cartridge, (b) drive the tone arm and (c) allow the tone arm cartridge system to respond to warps, to the extent the tracking force is used up in responding to a warp, insufficient tracking force will be available to accelerate the stylus, i.e., to enable the stylus to perform its primary function of tracking the signal modulation.
Merely increasing the tracking force to overcome the above problems results in excessive wear and can even collapse of the fragile stylus cantilever of many of today's better cartridges. Numerous techniques have, consequently, been suggested to damp the tone arm resonance and suppress the resonant peak in the response-frequency curve. Since the peak of the tone arm resonance can be reduced by increasing the resistance associated with the tone arm-cartridge system from its typical nominal value of approximately 250 dyne sec/cm to 1850 dyne sec/cm, and thus reducing the Q of the system from approximately 3.5 to 0.5, numerous techniques have been suggested to increase such resistance and lower the Q of the system.
One of such techniques has been to increase resistance associated with the stylus compliance. Since, however, such an increase would cause the cartridge body to move in synchronism with the stylus at high stylus velocity, and since an electrical output occurs only when the stylus moves relative to the cartridge body, there would be little or no output for heavily modulated musical passages, that is, signal output would roll-off with increasing stylus velocity, hence this technique is not viable.
Another approach to increase the resistance has been to damp the pivot around which the tone arm rotates, and thereby increase the resistance of the system. While this approach acts effectively to reduce the peak of resonance, all of the motion of the tone arm must be induced as a result of stylus motion with the result that the system now exhibits enhanced sensitivity to warp frequency below resonance. A recently proposed third approach to increase the resistance involves attaching to the tone arm a damper the bottom of which rides on the record surface. This damper should have a resistance of approximately 1850 dyne sec/cm and be partially supported by the tone arm and partially by the record surface. This approach has been described in an article entitled "A New Method of Arm/Cartridge Damping" by A. R. Rangabe, published in the collected Papers, 2nd. Edition, of the 1975 Convention of the Audio Engineering Society. The damper described by Rangabe consists of a support plate attached to the tone arm at the cartridge and a spigot attached to this plate and extending into a cup which is partially filled with a high viscosity silicone fluid and rests on the record surface. A compliant rubber diaphragm is bonded to the cup and the spigot, and a guide bearing is provided at the upper end of the cup to keep the cup and the spigot in alignment. An alternative embodiment using a solid visco-elastic damping material in place of the silicone fluid is suggested but not described.
While the prior art damper provides an approximation to the necessary damping action, and thereby reduces the Q of the tone arm resonance without requiring stylus motion, there are many advantages inherent in the present invention over the prior art damper. In increasing the system resistance to reduce the Q of the resonance, it is desirable to provide a damper which adds only a velocity dependent resistive loss to the system and which specifically does not appreciably add parasitic mechanical factors such as friction or springiness, because, if and to the extent present, extraneous mechanical factors complicate the mechanical response of the system, detract from the intended function of the damper and are detrimental to the performance of the system. The present invention however, does not introduce significant parasitic factors including the friction of the guide bearing, the stiffness of the diaphragm, the springiness of the air trapped between the silicone fluid and the diaphragm, and the friction at the cup-record interface due to a teflon (PTFE) coating since teflon does not exhibit its low-friction properties at the extremely low bearing forces appropriate to tracking a high fidelity record. Substitution of a visco-elastic damping material, while possibly eliminating the need for a guide bearing and its friction, merely substitutes the springiness inherent in all visco-elastic solids for the stiffness of a diaphragm and the springiness of trapped air.
Accordingly it is a general object of the present invention to provide an improved tone arm and damping system.
It is another object of the present invention to provide an improved damper for a tone arm-cartridge system which does not add appreciable springiness to the system.
It is a further object of the present invention to provide an improved damper for a tone arm-cartridge system which does not add appreciable frictional losses to the system.
It is yet another object of the present invention to provide an improved damper for a tone arm-cartridge system which has low mass and is not susceptible to fluid leakage.